Insulin resistance is a marker for increased risk of Non-insulin dependent diabetes mellitus, hypertension, and cardiovascular disease. The degree to which insulin sensitivity is determined by genetic and/or environmental factors is unknown. In order to define the heritability of insulin resistance, maximally insulin-stimulated glucose uptake will be measured using the glucose clamp technique in MZ and DZ twin paris who are free from overt disease. We have enrolled 92 subjects and are currently analyzing the data. We are writing a manuscript, and unless more subjects are needed, the study is complete.